Bound SQLite memory across the whole connection pool (closes #8)

The 3 GiB page cache and temp_store=MEMORY were set once in Initialize, so
only one pooled connection carried them and the other nine got no busy_timeout,
which drove many "database is locked" errors. None of that memory was visible
to the Go runtime.

Move the per-connection settings into the DSN so every pooled connection gets a
64 MiB cache (640 MiB worst case over ten connections), synchronous OFF, a
5 s busy_timeout and WAL. Drop cache_size, temp_store, synchronous and
busy_timeout from the Initialize pragmas; DISTINCT temp B-trees now spill to
disk. Add process-wide soft (1 GiB) and hard (1.5 GiB) heap limits; at the hard
limit a statement returns SQLITE_NOMEM and the existing batch paths already log
and drop, so nothing panics or exits.

Model: opus-4-8

internal/database/database.go

@@ -38,6 +38,22 @@ const (
 	maxIPv4        = 0xFFFFFFFF
 )
 
+// SQLite memory tuning. cache_size and busy_timeout go in the DSN so every
+// pooled connection gets them; the heap limits are process-wide and set once.
+const (
+	// sqliteCacheSizeKiB is the per-connection page cache; negative means KiB.
+	// -65536 = 64 MiB, so at most 640 MiB across the 10-connection pool.
+	sqliteCacheSizeKiB = -65536
+	// sqliteBusyTimeoutMs is how long a connection waits on a locked database.
+	sqliteBusyTimeoutMs = 5000
+	// sqliteSoftHeapLimitBytes (1 GiB) makes SQLite recycle its cache rather
+	// than allocate once its C heap passes this size.
+	sqliteSoftHeapLimitBytes = 1073741824
+	// sqliteHardHeapLimitBytes (1.5 GiB) fails a statement with SQLITE_NOMEM
+	// instead of growing the C heap without bound.
+	sqliteHardHeapLimitBytes = 1610612736
+)
+
 // Common errors
 var (
 	// ErrInvalidIP is returned when an IP address is malformed
@@ -71,11 +87,13 @@ func New(cfg *config.Config, logger *logger.Logger) (*Database, error) {
 		return nil, fmt.Errorf("failed to create database directory: %w", err)
 	}
 
-	// Add connection parameters for go-sqlite3
-	// Configure SQLite connection parameters
+	// Per-connection SQLite settings go in the DSN so every pooled connection
+	// gets them, not just the one that runs the Initialize pragmas.
 	dsn := fmt.Sprintf(
-		"file:%s",
+		"file:%s?_cache_size=%d&_synchronous=OFF&_busy_timeout=%d&_journal_mode=WAL",
 		dbPath,
+		sqliteCacheSizeKiB,
+		sqliteBusyTimeoutMs,
 	)
 	db, err := sql.Open("sqlite3", dsn)
 	if err != nil {
@@ -104,15 +122,17 @@ func New(cfg *config.Config, logger *logger.Logger) (*Database, error) {
 
 // Initialize creates the database schema if it doesn't exist.
 func (d *Database) Initialize() error {
-	// Set SQLite pragmas for performance
+	// Set SQLite pragmas for performance. Per-connection settings (cache_size,
+	// synchronous, busy_timeout, journal_mode) live in the DSN; temp_store is
+	// left at its default so DISTINCT temp B-trees spill to disk instead of C
+	// heap. The heap limits below are process-wide, so setting them once here is
+	// enough for the whole pool.
 	pragmas := []string{
 		"PRAGMA journal_mode=WAL",        // Write-Ahead Logging
-		"PRAGMA synchronous=OFF",         // Don't wait for disk writes
-		"PRAGMA cache_size=-3145728",     // 3GB cache (upper limit for 2.4GB DB)
-		"PRAGMA temp_store=MEMORY",       // Use memory for temp tables
-		"PRAGMA busy_timeout=5000",       // 5 second busy timeout
 		"PRAGMA analysis_limit=0",        // Disable automatic ANALYZE
 		"PRAGMA auto_vacuum=INCREMENTAL", // Enable incremental vacuum
+		fmt.Sprintf("PRAGMA soft_heap_limit=%d", sqliteSoftHeapLimitBytes),
+		fmt.Sprintf("PRAGMA hard_heap_limit=%d", sqliteHardHeapLimitBytes),
 	}
 
 	for _, pragma := range pragmas {